Quite often home improvement stores may carry a whiteboard that can be cut to size. Another option is to use a glossy white laminate but always test the pen out to ensure it wipes clean:)

Any sign shop or vinyl graphics business can get you vinyl to make custom white board / dry erase board to whatever size you need. It can be stuck to walls, drywall, etc. Be aware that stuck directly to painted drywall will destroy the dry wall if removed, will pull the paper face off the drywall.

Awesome cabinet for organization! Appreciate the thought and design that went into this! Great job!